A Ca-Ion Electrochromic Battery via a Water-in-Salt Electrolyte

Multivalent-ion batteries with electrochromic functionality are an emerging green technology for development of low-carbon society. Compared to Mg²⁺, Zn²⁺ and Al³⁺, Ca²⁺ has a low polarization strength similar to that of Li⁺, therefore Ca²⁺ for electrochromism and battery can avoid kinetic issues caused by other multivalent-ions with high polarization strength. Here, by exploiting Ca-ion carriers for electrochromism and a water-in-salt (WIS) Ca(OTF)₂ electrolyte for the first time, a new and safe aqueous Ca-ion electrochromic battery (CIEB) has been demonstrated. The WIS Ca(OTF)₂ electrolyte demonstrates enhanced anion-cation interactions and decreased water activity. Vanadium oxide (VO_x) and indium hexacyanoferrate (InHCF) films are respectively developed as anode and cathode because of their stable and high-rate Ca²⁺ insertion/extraction, as well as matched electrochromism. The CIEB demonstrates a stable and high-rate capability, a high energy density of 51.4 mWh m⁻² at a power density of 1737.3 mW m⁻², and a greenish yellow-to-black electrochromism. The presented results are beneficial for understanding redox kinetics in WIS electrolytes, and inspire researches on batteries and electrochromism with multivalent-ions.